The present disclosure relates to treatment of subterranean formations and, in specific examples, to methods, compositions, and systems that use hydrogels of graft polymers for diversion of acidic treatment fluids in subterranean formations.
The production of desirable fluids (e.g., oil and gas) from subterranean formations may often be enhanced by stimulating a region of the formation surrounding a well bore. Where the subterranean formation comprises acid-soluble components, such as those present in carbonate and sandstone formations, stimulation is often achieved by contacting the formation with a treatment fluid comprising an acid. These acid stimulation treatments are often referred to as “acidizing” the formation. For example, where hydrochloric acid contacts and reacts with calcium carbonate in a formation, the calcium carbonate is consumed to produce water, carbon dioxide, and calcium chloride. After acidization is completed, the water and salts dissolved therein may be recovered by producing them to the surface, e.g., “flowing back” the well, leaving a desirable amount of voids (e.g., wormholes) within the formation, which enhance the formation's permeability and may increase the rate at which hydrocarbons subsequently may be produced from the formation. One method of acidizing, known as “fracture acidizing,” comprises injecting a treatment fluid comprising an acid into the formation at a pressure above the fracturing pressure. Another method of acidizing, known as “matrix acidizing,” comprises injecting the treatment fluid into the formation at a pressure below fracturing pressure within the subterranean formation.
To enhance acidizing treatments, various additives may be added to the acidic treatment fluid. One such additive is a gelling agent which may, among other things, increase viscosity of the treatment fluid for improved diversion and particulate suspension, increase penetration into the reservoir by decreasing the reactivity of such fluid, reduce fluid loss, and/or reduce pumping requirements by reducing friction in the well bore. In some instances, the acidic treatment fluid may be self-diverting to further enhance the treatment. Among other things, a self-diverting acidizing treatment may effectively place the acid in a desired region within the subterranean formation, thereby creating a more optimal interaction of the acid with the acid-soluble components of the formation, which may create a desired network of channels that may penetrate deeper into the formation than a conventional acidizing treatment. One such self-diverting treatment fluid includes a crosslinkable gelling agent, a crosslinking agent, and a pH buffer to provide a crosslink within a certain pH range. A crosslinkable gelling agent comprising crosslinkable polyacrylamide-based polymers has been found to be useful in calcium carbonate formations. In such a treatment, as the acid reacts, the pH of the self-diverting treatment fluid increases, which causes the fluid to viscosify so as to form a gel that, inter alia, temporarily plugs the perforations or natural fractures accepting the most fluid flow. When the remaining treatment fluid encounters the gel, it is diverted to other portions of the formation. This process then may be repeated—as the treatment fluid is diverted, the acid creates another conductive void, and the treatment fluid is viscosified, diverts flow, and so forth. Once the treatment is complete, the viscosified treatment fluid may be “broken” by reducing its viscosity to a more readily pumpable level, so that the full productivity of the well can be restored.
Despite the advantages of using gelling agents in acid treatments, such treatments may be problematic. For example, conventional polymeric gelling agents may leave an undesirable residue in the subterranean formation after use. As a result, potentially-costly remedial operations may be required to clean up the surfaces inside the subterranean formation. Foamed treatment fluids and emulsion-based treatment fluids have been employed to minimize residual damage, but increased expense and complexity often result.